The present application relates to magnetoresistive random access memory (MRAM). More particularly, the present application relates to a double magnetic tunnel junction (MTJ) stack that improves the performance of spin-transfer torque (STT) MRAM.
Spin-transfer torque MRAM uses a 2-terminal device as is shown, for example, in FIG. 1A (bottom pinned MTJ stack) that includes a MTJ stack that contains a magnetic pinned (or reference) layer 10, a tunnel barrier layer 12, a magnetic free layer 14 and a capping layer 16. FIG. 1B shows an alternative MTJ stack (a top pinned MTJ stack) that can be used as a component of a STT MRAM. The MTJ stack of FIG. 1B includes a metallic seed layer 15, a magnetic free layer 14, a tunnel barrier layer 12, and a magnetic pinned layer 10. In the drawings, the arrow within the magnetic pinned layer shows a possible orientation of that layer and the double headed arrows in the magnetic free layer(s) illustrate that the orientation in those layers can be switched.
In the MTJ stacks shown in FIGS. 1A-1B, the magnetization of the magnetic pinned layer 10 is fixed in one direction (say pointing up) and a current passed down through the junction makes the magnetic free layer 14 parallel to the magnetic pinned layer 10, while a current passed up through the junction makes the magnetic free layer 14 anti-parallel to the magnetic pinned layer 10. A smaller current (of either polarity) is used to read the resistance of the device, which depends on the relative orientations of the magnetizations of the magnetic free layer 14 and the magnetic pinned layer 10. The resistance is typically higher when the magnetizations are anti-parallel, and lower when they are parallel (though this can be reversed, depending on the material).
In order to lower the switching current to flip the magnetic free layer relative to the magnetic pinned layer, a so-called double MTJ stack such as shown, for example, in FIG. 2, has been developed. The prior art double MTJ stack of FIG. 2 includes from bottom to top, a bottom magnetic pinned layer 20, a bottom tunnel barrier layer 22, a magnetic free layer 24, a top tunnel barrier layer 26, and a top magnetic pinned layer 28.
It is well established, that two magnetic layers (30, 34) can be coupled anti-ferromagnetically by separating them by a particular coupling layer with a very specific thickness (see, for example, FIG. 3). A typical coupling layer material for anti-ferromagnetic coupling is ruthenium (Ru). A specific thickness of Ru, which is often used for anti-ferromagnetic coupling, is about 4.5 Å or about 8.5 Å. Other example materials for the coupling layer for anti-ferromagnetic coupling include iridium (Ir) or rhodium (Rh). In the case of Ir, a thickness which is typically used is about 5 Å.
For highest anti-ferromagnetic coupling strength, the coupling layers need to have a strong texture with hexagonal symmetry with respect to the layer growth direction. The advantage of a synthetic anti-ferromagnet formed by coupling two magnetic layers through a Ru, Ir or Rh layer is the very low and controllable net-moment which consists of the difference of the two moments of the coupled layers. Such structures are particularly insensitive to external magnetic fields as long as these fields do not exceed strengths comparable to the anti-ferromagnetic coupling field of typically several thousands of Oe. While synthetic anti-ferromagnets have been widely used as magnetic pinned layers for prior MTJ stacks, synthetic anti-ferromagnets have been of less interest for magnetic free layers in prior MTJ stacks. This is because the combination of the need for hexagonal magnetic free layer symmetry for strong anti-ferromagnetic coupling conflicts with the need for cubic/amorphous symmetry for the magnetic free layer for high tunneling magneto resistance (TMR).
In regard to conventional double MTJ stacks, such as is shown, for example, in FIG. 2, the setting procedure for the magnetic pinned layers (20, 28) is challenging. In order to maximize spin transfer torque and minimize the write current both magnetic pinned layers (20, 28) need to be set in an anti-parallel configuration. This requires a sufficient window in coercivity allowing one to reach such a configuration by applying a two step field sequence. First, a strong field in one direction along the growth axis and then a second smaller field anti-parallel to the first field. If the magnetic free layer in a double MTJ stack is a synthetic anti-ferromagnet, the ideal magnetic pinned layer configuration for maximum spin torque is a parallel alignment of both magnetic pinned layers (20, 28).
Consequently, the coercivity window for top vs. bottom magnetic pinned layers (20, 28) requirement does not exist for double MTJ stacks with a synthetic anti-ferromagnet free layer. As such, there is a need for providing a double MTJ stack which provides a coercivity window for top vs. bottom magnetic pinned layers.